Allergy shots may be needed when you are exposed to a lot of allergens (e.g., pollen, dust mites) that you cannot avoid, or when allergy symptoms are so severe that allergy medications can't control them.

Allergy shots, or immunotherapy, are designed to desensitize your immune system to your allergy triggers or allergens. Allergy testing identifies these allergens. Allergy shots contain a very small amount of the allergen, and when you receive allergy shots, your body makes antibodies to the allergen. The next time you are exposed to the allergen, the antibodies block the effect of the allergen so that your symptoms are less severe. Over the course of your allergy treatment, your immune system builds up tolerance to the allergens.

An allergist, a doctor who specializes in allergies, will determine the appropriate amount of allergen for the allergy shot. The dose is individually prepared using standardized extracts of the specific allergen. For example, if you are allergic to grass, then standardized grass pollen will be used to prepare the allergy shots.

You need to get allergy shots about once or twice a week, over a period of several months. After about 3 to 7 months, your doctor may recommend that you start receiving allergy shots less often, called maintenance shots. Maintenance shots are given about once a month for 3 to 5 years. After this time, you may be able to stop having allergy shots.

Another type of immunotherapy uses modified extracts of allergens. These allergy shots are given over a shorter period of time and need to be given every year. For example, modified ragweed pollen can be given to people who have ragweed allergy before the ragweed season (which usually starts in mid-August).